In general, a spark plug used for ignition of internal combustion engines, such as automobile engines or the like includes a cylindrical metal shell, a cylindrical insulator disposed at the inner hole of the metal shell, a central electrode disposed at the inner hole in the leading end side of the insulator, and a ground electrode provided in a manner in which one end is in contact with the leading end side of the metal shell and the other end forms a spark discharge gap with the central electrode. Additionally, in the combustion chamber of an internal combustion engine, a spark plug brings about spark discharge at the spark discharge gap formed between the leading end of the central electrode and the leading end of the ground electrode, and combusts a fuel supplied in the combustion chamber.
As an electrode material of such a spark plug, a variety of Ni base alloys which are excellent in terms of oxidation resistance, spark corrosion resistance, or the like are widely used. For example, Patent Document 1 describes “Ni base alloy ignition plug electrode constituted by a Ni base alloy having a composition (hereinafter, % by mass) Cr: 0.5% to 5%, Mn: 0.1% to 3%, Si: 0.1% to 3%, Y: 0.00001% to 0.5% with the balance consisting of Ni and inevitable impurities.” Patent Document 2 describes “an electrode material for ignition plugs composed of, by % by mass, C: 0.1% or less (including 0), Si: 0.3% to 3.0%, Mn: less than 0.5% (including 0), Cr: less than 0.5% (including 0), Al: 0.3% or less (including 0) and a total content of 0.005% to 10% of one or two of Hf and Re with the balance consisting of Ni and inevitable impurities.” Patent Document 3 describes “an electrode for ignition plugs using a Ni base alloy including, by the mass ratio, Cr: 0.5% to 3%, Si: 0.3% to 2.5%, Mn: 0.5% to 1.8% (wherein 0.5% and 1.8% are not included) and Al: 0.05% to 2.5% (wherein 0.05% is not included) with the balance consisting of Ni and inevitable impurities, in which the ratio of Si to Cr (Si/Cr) is less than 1.1.”
However, in recent years, since there has been increasing demand for the prevention of global warming, conservation of fossil fuels, or the like, measures have been sought such as setting a large air-fuel ratio for fuel mileage improvement, or the like in the internal combustion engine of automobiles or the like. In such an internal combustion engine, there is a tendency that the temperature in the combustion chamber, in particular, near the area where the leading end of the central electrode and the leading end of ground electrode are located, is increased and the oxygen concentration in the combustion chamber is increased. Furthermore, since the sizes of the central electrode and the ground electrode are decreased due to the decreasing size of a spark plug, it becomes impossible to make the heat generated by discharge be transferred to the metal shell via the insulator and packing by the central electrode and to the metal shell by the ground electrode and then be removed (sometimes called heat dissipation), and therefore the temperatures at the central electrode and the ground electrode also become liable to increase.
If a spark plug is used in such an environment of a high temperature and a high oxygen concentration, and therefore the temperatures of the central electrode and the ground electrode are also liable to increase, it becomes difficult to maintain a desired performance in spark plugs of the related art. For example, there sometimes occurs a phenomenon called ‘pre-ignition’ in which a high temperature electrode acts as a source of ignition before regular ignition so that a fuel is ignited.